Method and apparatus for improving grain structures and soundness of castings



April 29, 1969 G. D. CHANDLEY METHOD AND APPARATUS FOR IMPROVING GRAINSTRUCTURES AND SOUNDNESS OF CASTINGS Filed Feb. 1, 1967 INVENTOR. 650266Q [AMA/0467 United States Patent ()7 3,441,078 METHOD AND APPARATUS FORIMPROVING GRAIN STRUCTURES AND SOUNDNESS F CASTINGS George D. Chandley,Alliance, Ohio, assignor t0 TRW, Inc. Cleveland, Ohio, a corporation ofOhio Filed Feb. 1, 1967, Ser. No. 613,313 Int. Cl. B2241 27/04, 23/00;B22c 9/02 U.S. Cl. 164--53 7 Claims ABSTRACT OF THE DISCLOSURE A castingprocess and apparatus utilizing porous ceramic shell molds in whichlayers of exothermic material are positioned around the molds and thenignited to provide preheated zones of different temperature in themolds, thereby preparing the molds for the reception of the molten metaland its solidification with directional orientation.

BRIEF SUMMARY OF THE INVENTION This invention is particularly intendedfor use in the casting of metals such as superalloys in vacuum or inair. In the practice of the invention, the ceramic shell mold is firstpreferably wrapped with a liquid and vapor barrier film and thendiscrete layers of a moldable exothermic material are packed around theshell mold structure. The exothermic material contains a hardenablebinder which, upon heating at relatively moderate temperatures,substantially below the casting temperature, hardens and provides aself-sustaining layer. Each exothermic composition has its owncharacteristic ignition temperature and the mass and dimensions of theexothermic layers are arranged so that the ceramic mold and their gatesand runners achieve a desired temperature upon ignition of theexothermic material. The shell molds thereby reach operatingtemperatures far faster than they would by other means of heating. Inaddition, the consolidation of the exothermic material makes it possibleto handle the molds more conveniently so that after ignition, it can bemoved into position over a heat abstracting surface whereupon the moltenmetal can be poured into the mold. The Zones of differing temperatureprovided by the ignition of the exothermic material, coupled with theuse of the highly heatconductive surface at the base of the mold providedirectional grain growth resulting in the preferred form of theinvention in a columnar structure being produced.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a cross-sectional view ofa molding assembly prior to the introduction of molten metal into thecasting cavities;

FIGURE 2 is a view similar to FIGURE 1 but illustrating the mold inposition to receive the cast metal; and

FIGURE 3 is a plan view of the interior of a vacuum casting chamberillustrating the manner in which the molds can be handled duringpreheating and casting.

DESCRIPTION Recent studies have shown that castings consisting of anoriented columnar structure have certain desirable properties inapplications such as jet turbine airfoils. The

methods heretofore employed for producing such structures, whileadequate, are costly due to the fact that they usually require severalhours time and employ expensive equipment. These long cycles also cancause excessive segregation in certain types of alloys, producingimperfect grain structures. These imperfect structures and misori entedgrain in general have been shown to be harmful in jet engine operation.

One of the objects of the present invention is to provide an improvedmethod for achieving directional orientation in cast structures, whereinthe time involved is substantially reduced from that required byprevious methods.

Another object of the invention is to provide a method for producingsound castings of airfoils and the like eliminating segregation defects.

Another object of the invention is to provide a method for producingcolumnar castings through the use of conventional vacuum melting andpouring equipment rather than with the use of more expensive, speciallydesigned equipment.

A still further object of the invention is to provide an improvedmolding assembly which can be handled without breakage at hightemperatures, thereby decreasing the time involved in the overallcasting process.

The present invention is particularly applicable to the use of porousceramic shell molds having a significant degree of gas permeability.Such molds are commonly produced by precision investment castingtechniques utilizing wax or wax and resin and mixtures as a disposablepattern material. One such method involves coating a disposable patternof wax or the like by dipping it in an aqueous ceramic slurry having atemperature about the same as that of the pattern material to form arefractory layer of a few mils in thickness. A typical slurry maycontain ceramic materials such as zirconium oxide, a binder such ascolloidal silica, and a thickener and low temperature binder such asmethyl cellulose. The initial layer while still wet is then dusted withsmall particles (40 to 200 mesh) of the refractory glass compositionsuch as that known as Vycor which is a finely divided, high siliconoxide glass containing about 98% silica and a small amount of boricacid, together with traces of aluminum, sodium, iron and arsenic. Thepattern with the dusted wet refractory layer on it is then suspended ona conveyor and moved to a drying oven having a controlled humidity andtemperature, thereby drying the coated pattern adiabatically.

The steps of dipping, dusting and adiabatic drying are then repeatedusing air at progressively lower humidities for succeeding coats. Forexample, the first two coats can be dried with air having a relativehumidity of 45 to The third and fourth coats can be dried with arelative humidity of 35 to 45%, the fifth and sixth coats, with arelative humidity of 25 to 30%, and the final coat with a relativehumidity of 15 to 25%.

The first layer is preferably applied to a thickness of 0.005 to 0.020inch, and the fine refractory particles are dusted onto the wet layerwith sufficient force to embed the particles therein. It is preferredthat the dusting procedure used provide a dense uniform cloud of fineparticles that strike the wet coating with substantial impact force. Theforce should not be so great, however, as to break or knock off the wetprime layer from the pattern. This process is repeated until a pluralityof integrated layers is obtained, the thickness of the layers each beingabout 0.005 to 0.020 inch.

After the mold is built up on the pattern material, the pattern can beremoved by heat, and then the green mold is ready for firing. Generally,firing temperatures on the order of 1500 to 1900 F. are used. Theresulting shell molds are hard, smooth, and relatively permeable andmeasure on the order of A; to A1 inch in thickness.

Referring now to the drawings, in FIGURE 1 there is illustrated acasting assembly in which a ceramictype shell mold 11 is disposed. Theshell mold 11 includes a pouring basin 12 communicating with a down gate13 which feeds runners 14 from which the molten metal is delivered tomolding cavities 16. In the particular embodiment illustrated in thedrawings, the molding cavities define airfoil shapes for jet engineblades. The molds have open ends which rest on a ceramic block 17. Aring support 18 is provided to facilitate lifting the mold assembly, aswill be apparent from a succeeding portion of the description.

Before the various layers of exothermic material are placed in the moldassembly, it is desirable to wrap the entire mold assembly 11 in aliquid and vapor barrier film such as polyvinylidene chloride whichprevents liquids from penetrating into the clean ceramic molding cavity.The wrapped mold assembly 11 is then positioned, as shown in FIGURE 1,and a first layer 19 of moldable exothermic material is applied to thedesired depth. Exothermic materials are available commercially for useat specified temperatures. They usually consist of a mixture of aluminumpowder and iron oxide in varying proportions, depending upon thetemperature to be achieved, together with a strong oxidizer such aspotassium perborate. In accordance with the present invention, thisexothermic material is mixed with a hardenable binder such as linseedoil to form a moldable composition which is then rammed into place toform the layer 19. Then, a supporting sleeve 21 is placed within thering support 18, and a second layer 22 of a moldable exothermiccomposition having a different characteristic burning temperature isrammed into the space between the mold assembly 11 and the sleeve 21.Finally, in the area of the pouring basin 12, a third layer 23 of amoldable exothermic material is rammed into the assembly to provide azone of different temperature along this portion of the mold. Thethickness of the exothermic materials, their composition and theirgeometry can be varied to secure quite accurately defined temperaturesin the various zones of the mold.

Next, the pouring basin 12 is covered with a ceramic cover 24, and thecomposite mold assembly is baked in an oven at about 400 F. to form alow temperature bond in the rammed powders by hardening of the linseedoil binder. This produces a self-sustaining structure which can then belifted without fracture by means of the support ring 18.

The exothermic layers are then ignited, whereupon they heat the moldsvery rapidly, in most cases less than onehalf hour, to providetemperatures suitable for directional solidification. When thetemperatures in the various zones of the mold have become fairly uniformwithin the zone, the mold assembly 10 is lifted and placed on arefractory block 26 in a vacuum chamber generally indicated at numeral27 in FIGURE 3. The refractory block 26 is supported on a turntable 28which also carries a pair of chill plates 29 and 31 consisting of ahighly heat-conductive metal such as copper and being preferablyprovided with a circulating coolant.

A vacuum is drawn through the vacuum chamber and the alloy to be pouredinto the mold is melted. Of course, for alloys which are capable ofbeing cast in air, the vacuum chamber would be unnecessary. When themolten metal is ready for pouring, the mold assembly 10 is raised andthe turntable 28 is rotated to place the chill block 29 under the moldassembly as illustrated in FIGURE 2.

Shortly after the mold assembly is placed on the chill block 29, themolten metal is poured in, and solidification begins.

The ignition of the exothermic layers 19, 22, and 23 drives out thebinder and leaves the layers in sintered form. These masses ofexothermic material protect the solidifying casting from air, so thatthe vacuum chamber may be opened shortly after pouring and a second moldassembly can be placed in the refractory block 26. Then, the process ofvacuum pumpdown and pouring can be completed while the previous castingis solidifying and cooling on the chill block '29.

The following specific example illustrates the process of the inventionmore specifically.

Example A six cavity ceramic mold was made up using the conventionalceramic mold process. The mold was uniformly wrapped with Saran Wrap(polyvinylidene chloride) to form a low temperature seal between theexothermic materials and the ceramic mold. A moldable exothermicmaterial providing a temperature of 3150 F. was rammed to a height ofabout two inches in the layer identified at reference numeral 19 ofFIGURE 1. This exothermic material included linseed oil as a binder.Then, material providing a temperature of 2950 F. was rammed into thespace to form a layer 22 as illustrated in FIGURE 1, and finallyexothermic material having a combustion temperature of 3150 F. wasrammed in the upper portion of the assembly to provide the layer 23,shown in FIG- URE 1. The mold assembly was then baked at a temperatureof 350 F. for twelve hours and was ignited by placing it in the furnaceat 1900 F. for three minutes. The mold assembly was then placed on aceramic block and the exothermic material was allowed to burncompletely. After complete ignition, the assembly was placed in a vacuumfurnace on a ceramic block such as block 26, illustrated at FIGURE 3. Anickel base superalloy was melted and brought to pouring temperature atwhich time the mold assembly was raised off the refractory block and awater cooled copper chill block, such as block 29, was rotatedunderneath the mold assembly, as illustrated in FIGURE 2. The metal waspoured into the mold at 2950" F., and the casting was allowed tosolidify and cool for thirty minutes, whereupon it was removed from thevacuum furnace. A metallographic cutup revealed excellent soundness andcolumnar grain structure.

It has been found that the method of the present invention reduces thetime cycles for the production of fully columnar castings to less thanabout 20% of the time required by previous methods using electricalheating. What is more, this method reduces cracking in airfoil typecastings in some high temperature alloys. Furthermore, segregationdefects in alloys have been found to be greatly reduced because of theshort solidification time. In addition, conventional vacuum melting andpouring equipment can be used which is far cheaper than that previouslyin use for making directionally solidified castings. As a furtheradvantage, since the temperatures are determined by pre-mixingexothermic materials, no constant temperature measurements are required.

It should be evident that various modifications can be made to thedescribed embodiments without departing from the scope of the presentinvention.

I claim as my invention:

1. The method of casting which comprises providing a ceramic shell moldhaving a molding cavity therein, molding a plurality of exothermiccompositions about said shell mold, each of said compositions having acharacteristic burning temperature, said compositions being positionedadjacent areas of said mold which are to be preheated to differenttemperatures, igniting said compositions while so positioned, andthereafter pouring molten metal into said molding cavity when saidtemperatures have been achieved.

2. The method of claim 1 in which heat is abstracted from the base ofsaid mold during and after pouring to provide a directionally solidifiedcasting.

3. The method of claim 1 in which said igniting is done while said moldis positioned on a heat insulating surface and said pouring is doneWhile said mold is positioned on a highly heat conductive surface.

4. The method of claim 1 in which said exothermic compositions contain abinding agent, and said compositions are preheated prior to ignition toform a self-sustaining mass.

5. A casting assembly comprising a porous ceramic shell mold having acasting cavity therein and a plurality of layers of exothermic materialpositioned about said cavity, each of said exothermic layers having acharacteristic ignition temperature.

6. The casting assembly of claim 5 which also includes a liquid andvapor barrier film interposed between said shell mold and saidcompositions.

UNITED STATES PATENTS 3,367,393 2/1968 Lenahan et al. 164-361 X FOREIGNPATENTS 212,984 1/1961 Austria.

I. SPENCER OVERHOLSER, Primary Examiner.

V. RISING, Assistant Examiner.

US. Cl. X.R.

